Luminaires are known that comprise a series of generally vertical, right-angle prisms for reflecting light from a centrally located lamp. The reflectors for these luminaries are made with transparent material (glass, acrylic, etc.) and typically have sets of longitudinal prisms running from top to bottom. The reflector typically has a desired overall contour provided by the series of prisms. In most cases the desired overall contour is dome-like, with an upper part of smaller diameter and a lower part of larger diameter.
Reflectors of the type having a prescribed overall dome-like structure with a series of circumferentially spaced prismatic reflectors on the exterior surface are known. The prismatic reflectors are formed of two, preferably perpendicular, faces with the intersections of the faces aligned in generally longitudinal directions with respect to the longitudinal axis of the luminaire. The prismatic reflectors are arranged such that the light passing through the interior surface of the reflector strikes the outer surface at near the critical angle whereby the light is reflected toward the interior of the reflector at an angle that results in its exiting the reflector.
Such luminaires are typically configured such that a light source is supported near an upper end of the reflector, which is open at the lower end opposite the light source to form an exit aperture. The reflector wall generally terminates at the open end in a flange having a width slightly greater than the thickness of the wall of the reflector. This flange is typically formed by a planar bottom surface oriented perpendicular to the longitudinal axis of the reflector, which renders it horizontal when the luminaire is in use. An example of such a prior art luminaire is that shown in U.S. Pat. No. 5,036,445 (Osteen). As used herein, “flange” refers generally to the bottom part of the reflector that typically projects slightly from the outer surface of the reflector but includes also structures that form the bottom edge of the reflector without projecting beyond the outer surface.
A problem with the prior reflectors of this type is that the some of the light entering the reflector wall through the inner surface becomes trapped between the inner and outer walls. That is, some of the light that passes through the inner face of the reflector is reflected by the outer prism faces but is not then transmitted back through the inner surface because it is reflected from the inner surface. This light reflected at the reflector-air interface becomes trapped by repeated reflection between the outer prism faces and the inner surface, much as light is trapped in a waveguide. When this phenomenon is combined with the dome-shape of a typical reflector, the result is that the trapped light eventually travels down the sides of the luminaire at small angles with respect to the vertical (nadir), which are high angles of incidence with respect to the inner surface. The trapped light is ultimately incident on the bottom flange of the luminaire at a small angle of incidence and often passes directly through the flange with little change in direction, creating a bright annulus of light at angles near nadir.
In the general case, this annulus of light passing through the flange is unwanted. One reason the annulus is undesired is that it is very bright and, thus, contrasts with the remainder of the light distribution. The annulus is bright because the direction of the light is near nadir and does not distribute into the luminaire's light pattern. Instead, the light is concentrated into a small solid angle.